How can I see a union's members?

In the main union window, press the Union button. In the next window you’ll see a list of members, their points and places in the ratings.
You can easily establish relations with union members by sending them gifts or visiting their Islands. All these actions can be carried out in the main window on the same tab. All you need to do is select either the gift or the house icon.
Also, the green indicator next to a player's name will help you understand which union members are currently online.